The investigators have shown previously that nitric oxide (NO), a lipophilic gas synthesized by NO synthase (NOS) from L-arginine, has important functions in neuroendocrine and behavioral osmoregulation, as well as blood pressure control. The experiments proposed in this application will clarify the site and mechanisms by which NO modulates the magnocellular neuroendocrine system. Since NO is synthesized in magnocellular neurons and cells in the supraoptic nuclei (SON) project almost exclusively to the posterior lobe of the pituitary gland to regulate neuroendocrine responses to fluid and electrolyte imbalances our studies will focus on the SON. The studies are designed to test two hypotheses: 1) NO synthesized locally in the SON during osmotic stimulation activates guanylyl cyclase to increase 3',5' cyclic guanosine monophosphate (cGMP) which results in 2) inhibition of angiotensin II and prostaglandin release in the SON to attenuate peripheral secretion of oxytocin. The specific aims are to determine: 1a) whether the activities of NOS (i.e. citrulline) and guanylyl cyclase (i.e. cGMP) increase in microdialysates of the SON during osmotic stimulation; 1b) whether inhibiting the production of NO with L-Name retrodialyzed in both SON prevents the increase of cGMP in microdialysates associated with increasing plasma levels of oxytocin during osmotic stimulation; 1c) whether retrodialyzing both SON with 1H-[1,2,4) oxodiazolo [4,3-a)quinoxalin-1-one (ODQ), a specific inhibitor of soluble guanylyl cyclase, decreases intranuclear production of cGMP and reproduces the effects of L-Name on plasma secretion of oxytocin during osmotic stimulation. 2a) whether ANG II and prostaglandin levels increase locally in the SON following retrodialysis with L-NAME during osmotic stimulation; 2b1) whether this neuroendocrine response to L-NAME can be prevented by losartan, an ANG II AT1 subtype receptor antagonist, or 2b2) indomethacin, an inhibitor of cyclooxygenase, the enzyme that converts arachidonic acid to prostaglandins. This research is important because it will further our understanding of the neurobiology of body fluid regulation by NO. Since NO is required for plasticity of neural systems involved in learning and memory, as well as participates in neurotoxicity, clarifying the mechanisms of action of this novel neurotransmitter may lead to new drug therapies selective for treatment of pathological disorders affecting water balance with minimal effect on other physiological responses.